The original publisher, Softdisk sold many of its assets to Flat Rock Software in the 2000s. Flat Rock sold off bits and pieces over the years, and the Catacomb games were one of the last things it held [1]. In 2017, I wrote a toy implementation of Catacomb 3D in WebGL [2]. Since the project used art that was under copyright, I contacted Richard at Flat Rock to ask permission to use it. That conversation eventually led to me deciding to buy the overall game ownership from him [3].
The most important factor is likely the current owner wanting to making sure it's going into the hands of someone who will do something caring with the rights.
I was looking into buying that IP a few years ago, and the asking price was affordable (I just didn't have the $ at the time so I went onto other projects). Glad to see someone doing something with it, I think it's perfect for a remake.
[1]: https://web.archive.org/web/20150101022225/http://www.flatro...
[2]: https://github.com/jayschwa/CatacombWebGL
[3]: https://web.archive.org/web/20170623155041/http://www.flatro...